The Interplay of Viral and Host Factors in Chikungunya Virus Infection: Targets for Antiviral Strategies - MDPI
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Review
The Interplay of Viral and Host Factors in
Chikungunya Virus Infection: Targets for
Antiviral Strategies
Kai Zhi Wong 1 and Justin Jang Hann Chu 1,2,*
1 Laboratory of Molecular RNA Virology & Antiviral Strategies, Department of Microbiology &
Immunology, Yong Loo Lin School of Medicine, National University Health System, 5 Science Drive 2,
National University of Singapore, Singapore 117597, Singapore; E0079517@U.NUS.EDU
2 Institute of Molecular & Cell Biology, Agency for Science, Technology & Research (A*STAR),
61 Biopolis Drive, Proteos #06-05, Singapore 138673, Singapore
* Correspondence: miccjh@nus.edu.sg; Tel.: +65-6516-3278; Fax: +65-6776-6872
Received: 29 March 2018; Accepted: 28 May 2018; Published: 30 May 2018
Abstract: Chikungunya virus (CHIKV) has re-emerged as one of the many medically important
arboviruses that have spread rampantly across the world in the past decade. Infected patients come
down with acute fever and rashes, and a portion of them suffer from both acute and chronic
arthralgia. Currently, there are no targeted therapeutics against this debilitating virus. One
approach to develop potential therapeutics is by understanding the viral-host interactions.
However, to date, there has been limited research undertaken in this area. In this review, we attempt
to briefly describe and update the functions of the different CHIKV proteins and their respective
interacting host partners. In addition, we also survey the literature for other reported host factors
and pathways involved during CHIKV infection. There is a pressing need for an in-depth
understanding of the interaction between the host environment and CHIKV in order to generate
potential therapeutics.
Keywords: chikungunya virus; host factors; potential therapeutics; interactions; antiviral; viral
structural proteins; viral non-structural proteins
1. Introduction
In recent years, chikungunya virus (CHIKV) has re-emerged as one of the many arthropod-borne
viruses (arboviruses) that can pose serious international public health threats [1,2]. CHIKV is an
Alphavirus that belongs to the Togaviridae family and is transmitted mainly by two species of
mosquitoes, namely, Aedes albopictus and Aedes aegypti [3]. Chikungunya comes from a Makonde
word that refers to the bent-up posture that the disease induces [4]. CHIKV can be classified into
three different lineages with distinct genotypes corresponding to their respective geographical
origins. They include the Asian, East-Central-South African, and West African genotypes [5–8]. One
unique feature, which distinguishes CHIKV from its arguably well-conserved alphavirus cousins, is
its high serum viral loads, which can exceed 109 virus particles/mL [9]. This remarkable feature allows
easy transmission of the CHIKV to any feeding mosquitos.
According to historical records, CHIKV is likely to have been present since 1779 [10]. However,
due to the similar clinical manifestations between CHIKV and dengue infections, CHIKV-infected
patients were likely to have been initially misdiagnosed as having dengue infection [10,11]. CHIKV
was first isolated and identified during an outbreak in Tanzania in 1953 [4]. Thereafter, many
epidemics and outbreaks were documented in a number of African countries in 1958 with thousands
of people being infected [12]. Soon after, cases of CHIKV infection mushroomed in many Southeast
Viruses 2018, 10, 294; doi:10.3390/v10060294 www.mdpi.com/journal/virusesViruses 2018, 10, 294 2 of 30
Asian countries from the 1960s [13]. Between 2005 and 2006, a major unprecedented epidemic of
CHIKV swept across many countries within the Indian Ocean territories, where CHIKV was not
endemic [13]. During that outbreak, one-third of the 785,000 residents of La Réunion were infected
with CHIKV, with a 0.1% fatality rate [14,15]. This ever-expanding geographical range of CHIKV
infection seemed unstoppable [16,17]. In 2007, autochthonous CHIKV cases were reported in Italy,
making the first instance of an outbreak in temperate regions [18]. Outbreaks and other
autochthonous transmission events were subsequently reported in many non-endemic regions like
Singapore from 2006 to 2015; France (West-French indies, Caribbean islands) from 2013 to 2015; Spain
and Senegal from 2014 to 2015; Argentina, The United States of America, and Kenya in 2016; and Italy
in 2010, 2014, and 2017 [19–24].
CHIKV-infected patients experience a sudden onset of fever of about 40 °C, together with the
trademark symptoms of intense muscle pain in the arms, calves, and thighs, as well as arthralgia in
the ankles, elbows, knees, and wrists within 2 to 12 days of infection [25]. This is due to CHIKV’s
ability to infect both the skeletal muscle progenitor cells and fibroblasts in the connective tissues of
muscles and joints, where a high density of nociceptive nerve endings reside [26–28]. In addition,
patients also suffer from maculopapular rash, nausea, vomiting, headaches, lymphopenia, and
moderate thrombocytopenia [29,30]. The more severe cases, though rare, involve the manifestation
of neurological complications. In addition, there are a few cases of CHIKV causing miscarriages and
neonatal complications like neonatal encephalopathy after maternal-to-fetal transmission [31–34].
The immunopathogenesis of CHIKV infection has been extensively reviewed by Burt and colleagues [35].
These acute symptoms usually resolve within two weeks. However, a significant portion of
patients experience persistent and/or recurrent joint pains for months or years after contracting
CHIKV [36,37]. The mechanism of the progression of the CHIKV disease to the chronic phase remains
poorly characterized. However, recent studies have shown that macrophages may play a role in the
chronic manifestations of CHIKV [38,39].
Despite the significant healthcare threat posed by the CHIKV, there are still no available vaccines
or therapeutics for CHIKV infections [1,35]. Patients are usually given analgesics and anti-
inflammatory drugs to relieve symptoms. Even though there are a number of anti-CHIKV
compounds being reported, precise mechanistic data of these compounds, as well as efficacy studies
in mouse models, are lacking. Ribavirin and chloroquine are the only two drugs that have been tested
in clinical trials [40]. Despite having promising in vitro data, chloroquine was found to be ineffective
in clinical trials [41,42]. On the other hand, Ribavirin was found to be effective in alleviating chronic
symptoms. However, the clinical trial cohort (20 patients) was too small to provide conclusive
evidence [43]. Therefore, there is a pressing need to identify and develop novel antivirals to combat
CHIKV infection.
Although attempts to develop drugs that specifically target viral proteins have proven to be
successful, the process is rather time-consuming and costly [44–48]. Furthermore, these compounds
were found to often display narrow spectrum activities [49]. One promising approach is to target host
factors that are known to be hijacked by viruses using either new compounds or to repurpose existing
approved drugs [50–52]. However, there are still many gaps in the current knowledge of basic
virology and replication of CHIKV, which poses difficulties for the discovery of anti-CHIKV
therapeutics. The purpose of this review is to provide an overview of the existing research thus far
on the interplay of viral and host factors during CHIKV infection. We hope this will guide further
research into potential druggable targets in the discovery of potential therapeutics.
2. CHIKV and Its Replication Cycle
CHIKV is an enveloped, spherical virus with a diameter of about 60–70 nm [53–55]. The CHIKV
genome consists of a single-stranded, positive-sense, linear RNA that is about 11,800 nucleotides long
[56,57]. The 5′ end of the positive-sense RNA genome possesses a 7-methylguanosine cap, and the 3′
end has a polyadenylated tail [58]. There are two open reading frames (ORF) found in the CHIKV
genome, one for the non-structural proteins, the other for the structural proteins. The first ORF
encoding the non-structural proteins (nsP1, 2, 3, and 4) makes up nearly two-thirds of the genomeViruses 2018, 10, 294 3 of 30
[56]. The remaining one-third portion encodes for the five structural proteins (capsid, E3, E2, 6K, and
E1) [56]. The CHIKV genome contains three non-translated regions (NTR) [56,58]. The 5′ NTR consists
of 76 nucleotides, while the 3′ NTR is composed of 526 nucleotides [59]. The remaining NTR, which
is 68 nucleotides long, is found between the two ORF and carries a promoter sequence that allows
the generation of the 26S subgenomic RNA that encodes all the structural proteins [59]. The CHIKV
genome is enclosed within the nucleocapsid core in the mature virion. The nucleocapsid, made up of
240 units of capsid proteins [56], is in turn surrounded by an envelope made up of a host-derived
lipid bilayer. The CHIKV envelope is studded with 80 sets of trimeric spikes, with each spike
containing three E1-E2 heterodimers, which mediate entry into host cells [60,61].
Upon binding to the host cell receptor via E2 protein, the CHIKV particle is endocytosed by the
host cell via the clathrin-mediated pathway in a process involving Eps15 (epidermal growth factor
receptor pathway substrate 15), although clathrin-independent entry has also been reported [62–66].
Within the early acidic endosome, the low pH environment initiates the fusion of the viral envelope
and the endosomal membrane in a process mediated by the E1 protein [63,65–68]. The membrane
fusion process occurs rapidly (within 40 s after endocytosed) in Ras-related protein 5 (Rab5)-positive
endosomes and is found to be highly dependent on the presence of cholesterol [66]. Thereafter, the
virus disassembles releasing the viral RNA genome into the cytosol [68,69]. By hijacking the host
translational machinery, the first two-thirds of the viral RNA is rapidly translated into a polyprotein
(P1234), which consists of all the four non-structural proteins [53,56]. However, this polyprotein
(P1234) makes up to only 10% of all the translated transcripts due to the presence of an opal stop
codon between the nsP3 and nsP4 gene [58]. The remaining 90% is made up of the P123 polyprotein.
The opal stop codon is believed to play a role in regulating the levels of the nsP4 by the alphaviruses
[70]. P1234 is subsequently cleaved in cis by the viral protease nsP2 yielding P123 and nsP4 [53]. This
allows the formation of an initial but unstable replication complex, which allows the synthesis of
negative-sense strand viral RNA. At this phase of the replication cycle, structures known as
spherules, which are derived from the plasma membranes, can be observed to be studded along the
plasma membrane. The unstable P123-nsP4 complexes are proposed to be localized near the neck of
these spherules, which functions in protecting the double stranded RNA intermediates from
detection and degradation [71–73].
As the infection progresses, the spherules become internalized and contribute to the formation
of a membranous structure called virus-induced type 1 cytopathic vacuoles (CPV-I) [72–74]. A recent
study by Thaa and colleagues showed that most of the spherules induced by CHIKV infection remain
at the plasma membrane, and that the internalization of the spherules is dependent upon the
activation of the phosphatidylinositol-3-kinase-Akt-mTOR pathway [75]. The CPV is a typical
membranous structure (derived from both endosomes and lysosomes) found in Alphavirus-infected
cells that allows active synthesis of both the negative-sense RNA intermediate and the positive-sense
viral RNA [72–74]. The accumulation of the P123-nsP4 complexes eventually crosses a certain
stoichiometric threshold concentration leading to the cleavage of P123 in trans, releasing nsP1 protein
[76,77]. This results in a more stable replication complex within CPV-I, made up of nsP1, P23, and
nsP4 [78–80]. A stable replication complex is subsequently formed upon the final cleavage of P23 into
nsP2 and nsP3 proteins. This induces a switch from the synthesis of the negative-sense RNA intermediate
to the synthesis of both the full-length viral genome and the 26S subgenomic RNA [74,81].
The 26S subgenomic RNA (which encodes only the structural proteins) is then translated into
structural polyproteins [56]. Once the full length capsid protein has been translated, it undergoes
autocleavage almost immediately, while the translation of the remaining structural proteins
continues [57,82–84]. In addition, upon its self-cleavage, the capsid protein is able to recognize the
full-length genomic viral RNA [82–84]. The capsid proteins then oligomerize, packaging the viral
genome into the nucleocapsid core [85,86]. Again, similar to the non-structural proteins, two types of
structural polyprotein products can be identified after the self-cleavage of the capsid protein. The
presence of the major structural polyprotein (consisting of E3, E2, 6K, and E1) and the minor one (10–
18%) (containing only E3, E2, and TF) have been attributed to the slippery codon motif (UUUUUUA)
found on the 6k gene, resulting in a −1 ribosomal frameshifting event [87,88]. These structuralViruses 2018, 10, 294 4 of 30
polyproteins are then directed by the signal peptide found on the N-terminus of the E3 protein to the
endoplasmic reticulum membrane, where they undergo complete cleavage into individual proteins
(pE2 (E3-E2), 6K or TF, and E1) by host proteases [89]. E1 and pE2 proteins associate noncovalently,
forming a heterodimer complex, which undergoes posttranslational modifications as it gets shuttled
via the Golgi secretory pathway. The host furin cleaves pE2, resulting in a mature E2 viral protein
[90–94]. Soon after, fully developed nucleocapsid cores carrying full-length, positive sense genomes
are recruited to the cell plasma membrane where they bud out of the host cells, simultaneously
acquiring a portion of the host plasma membrane studded with mature envelope glycoproteins,
making up the envelope of the mature virion [53].
It is also interesting to note that in the late phase of infection, another type of cytopathic vacuole
called CPV-II can be observed in infected cells [95–97]. In contrast to CPV-I, CPV-II originate from
the trans-Glogi network [98,99]. Within these vacuoles, the viral envelope glycoproteins (E1 and E2)
have been found to be arranged in a hexagonal lattice and packed in arrays of helical tube-like
structures [98,100]. In addition, nucleocapids have also been observed along the periphery of the
cytoplasmic side of CPV-II [96,98,100]. Given that these structures are in close proximity to the plasma
membrane, it is postulated that they aid in viral assembly and/or transport of the envelope proteins
to the plasma membrane and viral release via a second mechanism, exocytosis [98,101].
3. Interplay of Host Factors with CHIKV Structural Proteins
Structural proteins are known to be involved in processes like entry, fusion, uncoating of virus
particle, assembly of virions, and budding. Here, we present a brief review of the functions of each
individual viral proteins and their reported interacting host factors.
3.1. Capsid Protein
The CHIKV capsid protein is a compact multifunctional protein of 261 amino acids with a
molecular weight of about 30 kDa [58,102,103]. Unlike the New World encephalitic alphaviruses, the
capsid proteins of CHIKV (an Old World arthritogenic alphavirus) do not seem to be involved in the
shutting down of the host transcriptional processes [104]. Instead, the Old World arthritogenic
viruses rely on nsP2 for inducing host transcriptional and translational shutoff [104].
The capsid protein is made up of three main regions (regions 1, 2, and 3) [105,106]. Region 1
(1–80 aa) being highly basic in nature (Arg-, Lys-, and Pro-rich), is proposed to be able to bind RNA
in a non-specific manner and may also be involved in protein interactions that inhibit host
transcription [106]. In contrast, region 2 (81–113 aa) binds specifically to the full length viral RNA
genome and also plays an important role in the oligomerisation of other capsid proteins in order to
form mature nucleocapsid particles [85,106–109]. Region 3 is a serine protease containing a conserved
catalytic triad (His 139, Asp 161, and Ser 213) that is able to cleave itself in cis and inactivate itself by
binding its active site with its C-terminal tryptophan residue [53,82,83,110]. A recent study reported
that the CHIKV capsid is also able to exhibit trans-cleavage properties [111]. In addition, the
hydrophobic pocket (containing interacting residues: Val 130, Gly 131, Val 134, Met 135, Trp 245, and
Val 250) located on region 3 was found to interact with Pro 404 of the E2 cytoplasmic domain, which
is believed to occur during mature particle assembly [102,106,112]. Moreover, dimerization of two
capsid protein monomers relies on the interaction of the Tyr 186 residue from one monomer with two
Asn residues at positions 188 and 220 from the other monomer, all of which are located on region 3
[102].
The CHIKV capsid protein has been reported to possess both one nuclear import (NLS) and two
nuclear export signals (NES) (44–53 aa & 143–155 aa), which allows the protein to traverse actively
between the nucleus and cytoplasm [113,114]. In addition, two host proteins, karyopherin α (Karα)
and chromosomal maintenance 1 (CRM1), have been found to be involved in the active nuclear
import and export of the CHIKV capsid protein, respectively [113]. Interestingly, both NES are
required to be intact for CHIKV capsid to be exported. Mutation of the NES near the N-terminus (44–
53 aa), by replacing the Lys 51 and Met 53 with alanines, resulted in the retention of the CHIKV capsid
within the nucleus [114]. Unexpectedly, this lead to the blockage of the host nuclear import systemViruses 2018, 10, 294 5 of 30
through a mechanism which remains unknown [114]. In addition, Taylor and colleagues showed that
mutating the nucleolar localization sequence (NoLs) within the N-terminus results in an attenuated
phenotype with smaller plaques and reduced virulence in mice while still being able to elicit an
immune response [113–115]. The precise location of the NLS of the CHIKV capsid has yet to be
confirmed. Jacobs and colleagues suggested that the location falls between 1 and 83 aa of the capsid
protein, whereas Thomas and colleagues reported that it should be between 60 and 99 aa.
Given that the capsid protein is such a crucial, multifunctional viral protein with such an array
of functions, more efforts could be channeled to uncover the possible interactions with other potential
host factors.
3.2. E3 Protein
E3 proteins carry a signal peptide (a series of polar residues) at their N-terminus, which is crucial
for targeting the structural polyprotein towards the endoplasmic reticulum for initial processing
[53,106]. Despite being only 64 amino acids long (~7.4 kDa), it is necessary for the stabilization and
maturation of the E2 glycoprotein [58,87,116,117]. From crystal structures, the E3 protein was
observed to bind exclusively to the E2 protein [118]. It requires the host furin enzyme to mediate the
maturation of the E2 protein by cleaving it in the trans-Golgi system only after dimerization with
other available E1 glycoproteins is complete [106,118]. After cleavage, the E3 protein continues to
associate non-covalently with the E1-E2 spikes by relying on the interaction between the Tyr 47
residue on E3 and the Tyr 48 on the E2 [119]. It is not incorporated into the virus and will dissociate
when the entire complex is exposed to neutral pH at the plasma membrane surface [119]. Upon
dissociation, the acid-sensitive region between the E2 and E1 glycoproteins gets exposed, priming
the E1 protein for activation upon contact with low pH during entry [106,119]. E3 therefore plays an
important role in protecting the envelope glycoproteins from low pH and preventing their premature
activation. No other host factors that interact with the E3 protein have been reported.
3.3. E2 Protein
The E2 protein (423 aa, ~40 kDa), a type I transmembrane glycoprotein, has long been known to
be the main antigenic and receptor binding protein for CHIKV [58,120]. The CHIKV exhibits a wide
tropism by being able to replicate in many invertebrate and vertebrate cells [27,65,121–123]. In
addition, E2 proteins also serve as stabilizing factors (together with E3 proteins) for the E1-E2
heterodimer during the entire intracellular transport through the secretory pathway, where folding
and post-translational modifications take place, and finally to the plasma membrane [106].
To date, no bona fide receptor has been identified for CHIKV. However, there are host factors
that have been reported to aid in viral entry. For instance, prohibitin (PHB) was identified to aid in
the binding of CHIKV to CHME-5 microglial cells [124]. Wintachai and colleagues also suggested
that the possibility of additional co factors that assist the CHIKV entry as poor infection was observed
in U937 monocytic cells that also express PHB [124]. In a follow up study, Wintachai and colleagues
showed that flavaglines (prohibitin ligand) was able to inhibit CHIKV entry by preventing the
CHIKV and prohibitin from colocalising in HEK293T/17 cells [125,126]. These studies suggest that
PHB may be the putative receptor for CHIKV. In addition, ATP synthase β subunit was found to be
involved in the entry of CHIKV into Aedes aegypti mosquito cell lines [127]. Additional host factors
like protein tyrosine phosphatase non- receptor type 2 (PTPN2), fibril-forming collagen (COL1A2),
and actin gamma 1 (ACTG1) were also shown to interact with the E2 proteins in immunoprecipitation
experiments [128]. However, further mechanistic studies are required to understand the role of these
proteins during CHIKV infection.
Jemielity and colleagues showed that overexpression of either human T-cell immunoglobulin
and mucin-domain containing proteins 1 (hTIM1) or AXL receptor tyrosine kinase (also known as
UFO) of the TAM family of kinases promoted entry of CHIKV by at least 8-fold in HEK293T cells
[129]. Their work also showed that CHIKV entry is phosphatidylserine (PS)-dependent, as the PS
binding deficient hTIM1 variant did not support viral entry [129]. This suggests that PS is exposed
on the membrane of the CHIKV, similar to other enveloped viruses, including Pichinde virus,Viruses 2018, 10, 294 6 of 30
vesicular stomatitis virus, and vaccinia virus [130,131]. By taking advantage of the exposed PS, the
CHIKV is able to enter the cells. However, the authors noted that the blocking of hTIM1 receptors
was less effective in preventing CHIKV entry in Huh7 cells [129]. In addition, there are no reported
interactions between hTIM1 and the CHIKV receptor-binding protein, E2. This suggests that the PS-
recognising TIM and TAM receptors may not be the bona fide receptors for CHIKV but instead may
play a role as attachment factors that enhance CHIKV infectivity.
Recent work has shown that only the two surface-exposed domains (domains A and B) of
CHIKV E2 are able to bind to cells [132]. Binding of the CHIKV with soluble GAGs was found to be
able to inhibit CHIKV infection by up to 90% [132]. In addition, only domain A was able to bind to
cell-surface glycosaminoglycans (GAGs)-deficient cells, while domain B was found to interact
exclusively with cells expressing GAGs on their cell surface [132]. These results suggest that CHIKV
could employ more than one entry mechanism, which most probably explains the wide tropism of
cells observed. Therefore, more efforts could be directed to uncover other host factors/potential
receptors that could interact with the CHIKV E2 proteins.
3.4. 6K/TF Protein
The TF (~8.3 kDa) protein possess the same N terminus as 6 K (61 amino acids ~6.6 kDa) but
differs by having a longer basic C terminus (~15 residues) instead of a shorter, hydrophobic C
terminus found on 6 K [58,87,88]. Unlike other structural proteins, the exact functions of both 6K and
TF have not been clearly elucidated [133]. However, studies on other alphaviruses have suggested
that these viral accessory proteins mediate membrane permeability and viral budding, and may also
be involved in forming ion channels [87,134,135]. In addition, both 6 K and TF are also incorporated
in low levels into mature virions and are crucial in preserving both the stability and infectivity of the
virus [136–138]. However, no host factor has been found to interact with either of these proteins.
3.5. E1 Protein
The E1 protein (435 amino acids, ~44 kDa), a class II viral fusion protein, mediates the fusion of
the viral envelope with the host endosomal membrane after endocytosis [58,120,139]. This results in
the release of the nucleocapsid into the cytoplasm. A single mutation of Ala 226 to a valine residue
on the E1 glycoprotein enhanced the dissemination of CHIKV into the secondary organs of Aedes
albopictus mosquitos. Moreover, this phenomenon was also detected in the suckling mice [140]. This
mutation coincided with the emergence of Aedes albopictus as a second transmission vector during the
Indian Ocean epidemic in 2004 [15]. Moreover, in a recent study by Hoornweg and colleagues, this
mutation reinforced the cholesterol-dependent membrane fusion of the CHIKV with the host
endosomal membranes [66]. In another recent paper, another two epistatic mutations (E1:K211E
together with E2:V264A) were also found to notably enhance transmission (62 fold), infection
(13 fold), and dissemination (15 fold) in Aedes aegypti mosquitos [141]. However, the exact
mechanisms and possible interacting host factors that may facilitate the enhanced fitness of the virus
are still unknown.
In an attempt to uncover host proteins that interact with E1, Dudha and colleagues performed a
yeast two-hybrid (Y2H) screening on a human brain cDNA library [128]. The screen was able to
identify 5 interacting host proteins (copper metabolism (Murr1) domain containing 1 (COMMD1),
thrombospondin 1 (THBS1), dynein, cytoplasmic 1, heavy chain 1 (DYNC1H1), ATPase Na1/K1
transporting beta 3 polypeptide (ATP1B3), and microtubule-associated protein 1B (MAP1B)). Of
these, four hits (COMMD1, THBS1, DYNC1H1, and ATP1B3) passed and were validated via
immunoprecipitation and ELISA [128]. However, the biological significance and exact mechanisms
of the interactions have yet to be explored.
A second study reported that bone marrow stromal antigen 2 (BST-2 or tetherin or CD317) was
able to restrict and trap CHIKV on the surface of the host plasma membrane by engaging the E1
glycoprotein [142]. BST-2 knockout mice suggest that BST-2 is able to protect the lymphoid tissues
and regulate the inflammatory response induced by the CHIKV. Moreover, another study showed
that the longer isoform of BST-2 was found to specifically block the exit of alphaviruses (e.g., SFV andViruses 2018, 10, 294 7 of 30
CHIKV) efficiently [143]. In addition, although rubella virus and dengue virus share similar virion
structure as the alphaviruses, they responded differently to the presence of BST-2, with the dengue
virus not getting inhibited at all [143].
All in all, though much efforts have been focused on uncovering potential interacting host
factors, there are still many gaps in our knowledge understanding the interaction of the host factors
with the CHIKV structural proteins.
4. Interplay of Host Factors with CHIKV Non-Structural Proteins and Functions
The major function of CHIKV non-structural proteins (nsPs) is to replicate the viral genome for
translation of structural proteins and for packaging into progeny virions. Aside from this, most nsPs
also have additional functions outside of the replication complex. Here, we will present a brief review
of the functions of each individual viral protein and their reported interacting non-immunological
host factors.
4.1. Nonstructural Protein 1 (nsP1)
The nsP1 (535 amino acids, ~60 kDa) protein is responsible for the capping of both the positive-
sense genomic viral RNA and the 26S subgenomic RNA [58,144]. Interestingly, it caps the viral RNA
in a non-canonical manner where it first attaches a methyl group (hijacked from the host S-adenosyl-
methionine (AdoMet)) to a GTP before transferring the methylated guanylate residue to the nsP2-
processed 5′ end of the viral RNA [145,146]. For the capping to be successful, the triphosphates on
the 5′ end of the viral RNA need to be cleaved by the nsP2 triphosphatase, exposing the diphosphates
to allow the transfer to be complete [145]. Capping of the CHIKV RNA is believed to be a strategy to
confer protection against degradation by the host exonucleases and also enable efficient translation
of the viral mRNA.
To date, there is still no high resolution structural information on the nsP1 protein [147].
However, it has been suggested that the capping domain spans across at least the first 400 aa residues
from the N terminus [148]. With reference to the secondary structure of the nsP1 of Sindbis virus
(SINV), a related alphavirus, the CHIKV nsP1, has been speculated to carry guanylyltransferase
activities [149]. However, it is important to note that the sequence (or structural) homology between
SINV and CHIKV nsP1 is low, and a crystal structure and further confirmatory biochemical assays
would be needed for confirmation of the guanylyltransferase activity of CHIKV nsP1. Another
important function of the nsP1 protein is its monotopic interaction with the cytoplasmic side of the
plasma membrane bilayer, mediated by its amphipathic alpha helix (approximately between 244 and
263 aa), discovered in a relatively well-studied, close alphavirus relative, the Semliki Forest virus
[150,151]. Additionally, covalent palmitoylation of the nsP1 (417–419 aa) was discovered to be able to
strengthen the association with the plasma membrane [149]. This crucial interaction allows the nsP1
to direct and anchor the replication complex to the cell plasma membrane [152]. The interactions
between the nsPs have not been well explored. However, there are studies that have shown that nsP1
strongly interacts with nsP4 [153–155]. Similarly, there is also a lack of studies on the interaction with
host proteins. The BST-2 protein was identified to be an anti-viral factor that was downregulated by
the nsP1 protein to allow release of the viral particles tethered to the cell surface [142,143,156].
4.2. Nonstructural Protein 2 (nsP2)
The nsP2 protein is one of the more well-studied and also the largest nsP, consisting of 798 amino
acids with an approximate molecular weight of 90 kDa [58,144]. So far, there are no reports of high
resolution crystal structures of the entire CHIKV nsP2. However, crystals structures of the CHIKV
nsP2 C terminus (~471–791 aa) (PDB code 3TRK (2011) and 4ZTB (2016)) and that of alphavirus
relatives, SINV, and Venezuelan equine encephalitis virus (VEEV) are available for comparison [157–
159]. The domains in the N terminal region, however, have been proposed via molecular modelling
[160]. Starting from the N terminus, the nsP2 is hypothesized to have three structural domains [160].
The first domain (~1–167 aa) has little homology with other alphaviruses and remains unknown [160].Viruses 2018, 10, 294 8 of 30
The subsequent two domains (~168–470 aa) possess characteristic RecA-like domains of superfamily
1 (SF1) group of helicases [160]. The remaining portion (~471–791 aa) holds the protease domain,
which can be separated into two smaller sections. The first section bears a papain-like cysteine
protease domain (~471–605 aa), while the last section was appears to be a non-functional Ftsj (rrmj)
methyltransferase-like domain (~606–791 aa) [160].
This multi-functional nsP2 is capable of performing at least four enzymatic functions. The
helicase activity is only functional when the full length nsP2 is available, even though the effector
domains are found on the N terminus [160]. Truncated recombinant proteins were found to be
inactive [161]. The CHIKV nsP2 is only able to unwind double-stranded RNA that have a 5′ overhang
of at least 12 bp in and also only in the 5′–3′ directionality [160]. Since it possesses RecA-like domains,
it is not surprising that it facilitates the complementary base-pairing of single-stranded RNA [160].
The CHIKV nsP2 has also been reported to display nucleotide triphosphatase (NTPase) activities and
is able to hydrolyse all dNTPs and NTPs without any preference. However, this is observed only in
full length nsP2 [160,161]. Furthermore, the activation of the NTPase of the nsP2 by RNA and DNA
oligonucleotides is, again, only possible with the full length nsP2 [160]. Interestingly, the same active
site used for the NTPase activity has also been found to perform RNA 5′triphosphatase activities, the
third function, which mediates the cleavage of γ-phosphates from RNA substrates [161]. The last
known enzymatic function is the protease activity of the nsP2, which is responsible for processing of
the nsP polyprotein [162,163].
Apart from these enzymatic functions, the nsP2 has been long known to enter the host nucleus
during infection, albeit with the absence of any putative nuclear localization signals [147,164].
Although the exact mechanism is still unknown, the localization of nsP2 is essential in inhibiting the
host antiviral responses [165–167]. One interesting observation reported by Fros and colleagues is
that CHIKV nsP2 can be detected in the nuclear region at 4 h post infection (h.p.i) before translocating
to the cytoplasm at a later time point of 12 h.p.i [168]. One way that the CHIKV nsP2 is able to induce
the shutdown of host transcription is by mediating the degradation of a catalytic subunit of the RNA
polymerase II, Rpb1 [169]. The nsP2 protein has also been found to be responsible for the suppression
of the host cellular translation processes without affecting viral protein translation [53]. Interactions
of the alphavirus nsP2 protein with a large number of ribosomal proteins and proteins that are
involved in translations have also been revealed via immunoprecipitation assays [170]. For instance,
in VEEV, the ribosomal protein S6 (RpS6) was found to immunoprecipitated with nsP2 in both
mammalian and insect cells [171]. Low levels of RpS6 proteins have been correlated strongly with
diminished cellular translation activities [171]. However, the exact mechanism of this
dephosphorylation phenomenon is still unknown. Recently, nsP2 and nsP3 were also discovered to
exhibit RNA interference activity [172].
The promiscuous alphavirus nsP2 is able to bind to not only the other nsPs but also a number of
host factors. The interactions with nsPs were confirmed for CHIKV through immunoprecipitation of
GST and streptavidin-tagged nsPs and their various domains, and validated via a yeast two-hybird
screen and ELISA [173,174]. In addition, using computational techniques, Rana and colleagues were
able to propose a spatial model of the late CHIKV replication complex [173]. Bourai and colleagues
performed an extensive study on the host factors interacting with the nsP2 using a yeast two-hybird
screen [175]. While they identified 22 unique hits, only heterogeneous nuclear ribonucleoprotein K
(hnRNP-K) and ubiquilin 4 (UBQLN4) were observed to play a significant role in CHIKV replication
upon gene silencing [175]. Another host factor, the NDP52 human autophagy receptor, was shown
to interact with CHIKV nsP2 and acts as a pro-viral factor in human cells [176,177]. Although the
precise mechanism is still unknown, it has been postulated that the human NDP52 interacts with
nsP2 in the cytoplasm to prevent the latter from localizing into the nucleus. This could in turn delay
cell death, which would allow more time for the CHIKV to replicate [147]. The nsP2 is also implicated
in the suppression of the unfolded protein response (UPR) triggered by the production of CHIKV
envelope proteins in the endoplasmic reticulum (ER) [168]. Although the exact mechanism of action
remains to be clarified, incomplete splicing of the X-box binding protein 1 (XBP1) mRNA (complete
splicing is required for UPR) has been observed in cells transfected with nsP2 [168].Viruses 2018, 10, 294 9 of 30
Being such a multifunctional protein that is involved in a number of important processes such
as viral genome replication and host evasion, CHIKV nsP2 presents an intriguing and promising
target for drug development. This has being aptly reviewed by Bakar and Ng [178].
4.3. Nonstructural Protein 3 (nsP3)
The nsP3 (530 amino acids, ~60 kDa) protein possesses three domains, a highly conserved N-
terminal macro domain (~160 aa), followed by a zinc 2+ binding domain (~165 aa) and ends with a
variable “tail” region (~205 aa) [58,144,147]. The macro domain contains a ubiquitous protein module
found in all living organisms including positive sense RNA viruses like alphaviruses, coronaviruses,
hepatitis E virus and rubella virus [179]. The macro domain is believed to function mainly as an ADP
hydrolase that removes mono or poly ADP-ribose marks on proteins. ADP-ribose marks usually
occur on Asp, Glu, and Lys residues and are indicative of post-translational modifications by the poly
(ADP-ribose) polymerases (PARPs) [180,181]. In experiments involving mass spectrometry,
McPherson and colleagues discovered that the CHIKV nsP3 is able to recognize and hydrolyse the
ADP-ribose groups from mono ADP-ribose marks that found on only Asp and Glu residues [182].
The authors went on to show that the loss of the hydrolase significantly compromised the ability of
CHIKV to replicate in both baby hamster kidney and Aedes albopictus cells [182]. Moreover, CHIKV
infectious clones encoding hydrolases with impaired activities were found to replicate more slowly
in mouse neuronal NSC-34 cells with a significant decrease in fitness in neonate mouse model [182].
These results indicate that the macro domain of nsP3 proteins plays an important role in the viral
replication and virulence of the CHIKV. In addition, it also suggests that the levels of ADP-
ribosylation could play a major role in the anti-viral response by the host.
The zinc binding domain of CHIKV nsp3 was inferred from the crystal structure of the SINV by
Shin and colleagues [158]. Mutations in this region resulted in the impairment of both the negative-
sense and 26S subgenomic RNA synthesis and polyprotein processing in SINV [183,184]. Similarly,
mutations in the zinc-binding region of SFV nsP3 resulted in defects in neurovirulence [185].
However, little is known about their precise mechanisms.
The stretch of conserved residues (proline–rich) on the variable “tail” region was found to be
able to bind to the host amphiphysin1 and 2 proteins through the Src-homology 3 (SH3) domains
found on the amphiphysin proteins [186,187]. This phenomenon is observed readily upon the
transfection of the CHIKV nsP3. Although no validation was performed using CHIKV infected cells,
both SFV and SINV infected cells confirmed this observation [186]. Amphiphysins are postulated to
be involved in the formation of the spherules (which house replication complexes) given their ability
to induce membrane curvature [147,186,188].
The nsP3 also binds to GTPase-activating protein (SH3 domain)-binding protein 1 (G3BP1) and
its homolog (G3BP2). The G3BPs are found in stress granules, which are believed to have anti-viral
properties. For instance, stress granules are involved in the inhibition of RNA translation [147]. By
sequestering the G3BPs, the nsP3 is able to prevent stress granules from forming during infection
[189–191]. However, the depletion of the G3BPs was found to be unfavorable for CHIKV replication
[192–194]. The proposed pro-viral activity of the G3BPs includes aiding the switch from the
translation of non-structural proteins to viral RNA replication [192–194]. In mosquito cells, the G3BP
homolog, Rasputin (Rin), was found to also exhibit similar phenomenon [195]. However, in cell
culture, the silencing of Rin did not adversely affect CHIKV replication [195]. Instead a significant
reduction in total infectious viral titer was observed in Aedes albopictus in vivo [195]. Interestingly,
Remenyi and colleagues also showed that nsP3 is closely associated with the host cellular lipid
droplets and also with nsP1 [196].
Sphingosine kinase 2 (SK2) was reported to co-localize with nsP3 in nsP3 overexpression studies.
In addition, SK2 also colocalizes with the CHIKV RNA [197]. Moreover, Reid and colleagues showed
that upon SK2 knockdown, there was a significant reduction in infectious viral titer, suggesting a
pro-viral role [197]. However, the exact mechanism still remains to be solved. Additionally, there are
other host factors (Y-Box-Binding Protein 1 (YBX1), PI3K-Akt-mTOR pathway, DDX1/DDX3, and
IKKβ) that were reported to be able to bind or interact with the alphavirus (SINV, SFV, and VEEV)Viruses 2018, 10, 294 10 of 30
nsP3 reviewed Lark and colleagues [198]. However, these have not been validated for CHIKV.
Another pro-viral host protein found to co-immunoprecipitate with the nsP3 is the heat shock protein
90β (Hsp90β), but its exact function is unknown [199].
4.4. Nonstructural Protein 4 (nsP4)
The bulk of the nsP4 (611 aa, ~70 kDa) protein consists of an RNA-dependent RNA polymerase
(RdRp) (~500 aa), with the characteristic GDD motif, responsible for viral RNA synthesis at its C-
terminus [58,144,147]. The remaining ~100 aa at its N-terminus, despite being a relatively unknown
and seemingly intrinsically disordered stretch of sequences, is still important for the normal function
of the nsP4 in SINV [200]. To date, high resolution structures of the CHIKV RdRp are still not
available. Recently, Chen and colleagues were able to generate a truncated but soluble, well-folded,
functional RdRp catalytic subunit from E. coli [201]. They showed that the CHIKV RdRp has a
preference for single-stranded RNA with a 5′-overhang that is of at least 4 nucleotides long [201]. In
addition, the RdRp was also found to be rather sensitive to a number of detergents in comparison to
the relatively resilient Dengue virus RdRp [201]. The RdRp was also able to exhibit primed extension
of templates and terminal adenylyltransferase (TATase) activity regardless of the presence of any
template [201].
Similar to the nsP3 protein, the nsP4 protein was found to interact with another Hsp90 protein,
Hsp90α. Inhibition of Hsp90α resulted in a decrease in both viral RNA and protein levels [199].
However, the mechanism of the interaction remains to be discovered. In another study, Rathore and
colleagues showed that overexpression of the CHIKV nsP4 was able to suppress the phosphorylation
of eukaryotic translation initiation factor, alpha subunit (eIF2α), which in turn antagonizes the host
unfolded protein response during infection [202]. On the other hand, during SINV infection, rapid
phosphorylation of eIF2α was observed instead. However, the mechanism behind this interaction
remains unexplored.
An in-depth review on the nsP4 functions and interactions was also recently published by Pietila
and colleagues [203].
5. Other Host Factors That Are Involved in CHIKV Replication Cycle
In concert with the above efforts, a number of host factors have been identified via different
screening methods. For instance, Paingankar and colleagues discovered that CHIKV interacts with
housekeeping proteins like that actin, heat shock protein 70 (HSP70) and STAT2 (Vero-E6 cells only)
via virus overlay protein binding assay (VOPBA) complemented with matrix-assisted laser
desorption/ionization time of flight analysis (MALDI TOF/TOF) [204]. They went on to show that the
silencing of HSP70 resulted in a significant decrease in total infectious viral titer. However, the exact
mechanism remains to be solved.
Treffers and colleagues employed the use of stable isotope labeling with amino acids in cell
culture (SILAC) coupled with liquid chromatography–mass spectrometry (LC-MS) to uncover the
temporal dynamics of the cellular response during CHIKV infection [205]. They were able to pick up
13, 38, and 106 proteins that were differentially expressed at 8, 10, and 12 h.p.i, respectively.
Moreover, majority of the proteins detected were the subunits of RNA polymerase II, and they were
found to be progressively degraded [205]. This is in line with the observation that cellular
transcriptional shut off occurs during CHIKV. The authors also reported four anti-viral factors (Rho
family GTPase 3 (Rnd3), DEAD box helicase 56 (DDX56), polo-like kinase 1 (Plk1), and ubiquitin-
conjugating enzyme E2C (UbcH10)), which were down regulated during CHIKV infection [205].
However, the exact mechanisms remain to be discovered.
A very extensive human whole genome siRNA mediated loss-of-function screen was recently
performed in a bid to identify effective therapeutics against CHIKV [206]. Karlas and colleagues were
able to capture 156 pro-viral and 41 antiviral host factors that affect CHIKV replication [206]. They
performed pathway analysis of the identified pro-viral factors and subsequently identified 21 FDA-
approved small-molecule inhibitors that were effective against CHIKV by cross referencing with
specialized databases [206]. These 21 antiviral compounds were found to act on four host factorsViruses 2018, 10, 294 11 of 30
(vacuolar-type H+ ATPase (vATPase), CDC-like kinase 1 (CLK1), fms-related tyrosine kinase 4 (FLT4
or VEGFR3), and the K (lysine) acetyltransferase 5 (KAT5 or TIP60)) and two pathways (calmodulin
signalling and fatty acid synthesis) [206]. Through in vivo and in vitro work, three of the inhibitors
(Tivozanib, Pimozide, and 5-tetradecyloxy-2-furoic acid (TOFA)) were reported to exhibit
prophylactic antiviral effects in mice [206]. In addition, when the authors combined two inhibitors
(Pimozide and TOFA), each targeting the calmodoulin signalling and fatty acid synthesis pathways,
respectively. The synergistic effect resulted in a therapeutic antiviral effect in both in vivo and in vitro
studies [206]. However, the exact mechanism and role played by these host pathways in CHIKV
replication warrants further work. Nonetheless, the work by Karlas and team showed the importance
and relevance of understanding the interplay of host factors during viral infection, as well as the
significant translational value that can be gained from performing basic research on the importance
of host factors during CHIKV infection.
6. Conclusions and Perspectives
Even though a large number of host factors have been identified through these studies (Table 1),
the mechanistic details of the interplay of the host factors during CHIKV infection are still lacking.
The advantages of targeting host factors are plenty, as opposed to targeting just viral proteins. For
instance, targeting host factors may allow inhibition of a broad-spectrum of viruses that share same
host factors (Table 1). Moreover, those drugs could also be tested for synergistic effects with specific
viral protein inhibitors for development of a more comprehensive treatment plan that targets
multiple pathways. This therapeutic approach would also prevent the development of antiviral
resistance. For instance, small molecule inhibitors that mimic the interaction sites found on GAGs,
PHB, TIM, and TAMs could be developed. These would bind to CHIKV, drastically reduce the
opportunity for the CHIKV to interact with the attachment factors present on the host cells and hence
dampen its infectivity. Similarly, this approach could also be extended to host factors that are
involved in the CHIKV replication cycle like the NDP52 host protein and given in a cocktail to
patients.
A promising cocktail candidate for clinical trials would be the combination of Pimozide and
TOFA concocted by Karlas and team. In drug research, though many drugs screened may have
shown to possess great efficacy in vitro, many of them failed during the in vivo validation processes.
On the other hand, the cocktail of Pimozide and TOFA was able exhibit impressive efficacy in both
in vitro and in vivo studies. Therefore, we feel that there is great potential in this combination host-
targeting drug therapy.
It should still be noted that targeting host factors comes with the risk of toxicity, especially when
these host factors perform vital functions in the host cells. Design of therapeutics therefore needs to
be optimized to target interactions between host and viral factors, while minimizing disruption of
essential cellular processes. With the significant bottlenecks in our knowledge of basic CHIKV
virology and its interacting host partners, more research is needed to understand the molecular
mechanisms of the interactions between the CHIKV and its host factors. This would not only help to
increase the knowledge pool but also provide more opportunities and avenues to develop, optimize,
and/or speed up the production or repurposing of potential therapeutics to combat this medically
important re-emerging arbovirus.Viruses 2018, 10, 294 12 of 30
Table 1. Summary of host factors known to interact with CHIKV proteins.
Viral Validated Interacting Putative Function/Interaction with Examples of Interaction with Other
Known Functions of Host Proteins References
Protein Host Factors/Pathways CHIKV Postulated by the Authors Medically Important RNA Viruses
Karα4 (KPNA4): Karα4 (KPNA4):
A group of proteins that transport Karα4 (KPNA4): Proposed to Interact with Middle East
Karα4 (KPNA4) molecules between the cytoplasm and Binds to NLS of CHIKV capsid Respiratory Syndrome (MERS) virus protein [113,207]
nucleus. Able to act as either importins or protein for nuclear translocation. OF4b to prevent NF-kappa-B complex from
exportins. entering the nucleus.
CRM1 (XPO1):
Capsid Proposed to bind and export the following
CRM1 (XPO1): CRM1 (XPO1): RNA-containing viral proteins from the
Major mammalian export protein that Pro-viral factor. Binds to the NES/of nucleus to the cytoplasm: human
CRM1 (XPO1) [113,208–213]
facilitates export of RNA and proteins capsid, allowing exit from the immunodeficiency virus (HIV) Rev protein
from the nucleus to the cytoplasm. nucleus. cargo complex, human T-cell leukemia virus
type 1 (HTLV-1) rex protein, and influenza A
ribonucleoprotein complexes.
Furin: Furin:
Furin:
Calcium-dependent serine endoprotease. Shown to be essential for H5N1, H7N1 avian
E3 Furin Cleaves the E3 protein away from [106,118,214]
Preferentially cleaves at sites with paired influenza viruses, and Canine distemper virus
the precursor E2 polyprotein.
basic amino acids. (CDV). Actual mechanism unknown.
PHB:
Shown to interact with HIV-1 glycoprotein,
and the binding is important for its replicative
PHB:
spreading in cells.
Many reported functions, including PHB:
PHB Interacts with dengue virus E protein and is [124,215–217]
modulation of transcription and Possible attachment/entry factor.
the first characterized receptor protein for
chaperone functions in the mitochondria.
dengue virus in insect cells.
Proposed to be entry factors for hepatitis C
virus.
PTPN2:
E2
A tyrosine phosphatase that PTPN2: PTPN2:
dephosphorylates protein tyrosine kinases Postulated to be involved in Hepatitis C virus nonstructural 3/4A protease
PTPN2 [128,218]
in both nuclear and cytoplasm transportation of viral structural cleaves PTPN2 that induces a shift from host
compartments. Involved in numerous proteins to host plasma membrane. Th1 to Th2 immune response.
signaling events (e.g., endocytic recycling).
COL1A2:
COL1A2:
COL1A2: Shown to increase infectious viral titer of
COL1A2 A group 1 collagen found in most [128,219]
Mechanism unknown. Sindbis virus (SINV) and also proposed to aid
connective tissues.
in its transmission.
ACTG1 ACTG1: ACTG1: ACTG1: [128,220]Viruses 2018, 10, 294 13 of 30
Part of cellular trafficking machinery. Postulated to be involved in The human immunodeficiency virus type 1
transportation of viral structural (HIV-1) protease was found to cleave actin
proteins in host cell. (ACTG1).
GAGs: GAGs:
A group of complex linear polysaccharides Allows binding and infection of hepatitis B
expressed on cell surface, in intracellular virus.
GAGs:
GAGs compartments, and also in the Attachment factor for respiratory syncytial [132,221–225]
Possible attachment/entry factor.
extracellular environment, where they are virus (RSV), coronavirus NL63 (CoV-NL63),
able regulate many cellular processes and the severe acute respiratory syndrome
including (examples cell signaling, etc.). coronavirus (SARS-CoV).
hTIM1: hTIM1:
Involved in regulation of both innate and Implicated as receptors for non-enveloped
hTIM1:
hTIM1 adaptive immune responses, engulfment hepatitis A virus and enveloped viruses such [129,226–228]
Possible attachment/entry factor.
of apoptotic cells, and T cell— as Zaire Ebolavirus and Lake Victoria
proliferation. Marburgvirus.
AXL:
AXL:
Regulate and involved in many important
AXL receptor tyrosine AXL: Implicated as receptors for Ebolavirus,
physiological processes like cell [129,229–231]
kinase Possible attachment/entry factor. Marburgvirus, pseudo-typed lentiviral,
proliferation, survival, differentiation, and
vaccinia virus, and Lassa virus.
migration.
6K/TF - - - - -
COMMD1:
A proposed scaffold protein that is COMMD1:
COMMD1:
involved in diverse physiological Postulated to be involved in
Enhances latent infection of HIV-1 by
COMMD1 processes. Able to regulate the transport of viral structural proteins [128,232,233]
stabilizing IκB-α, the inhibitor of NF-κB, and
ubiquitination and degradation of specific in host cell and/or involved in
attenuating innate immune response.
cellular proteins including NF-κB subunit regulating host immune response.
p65.
THBS1:
THBS1:
Adhesive glycoprotein that binds heparin.
THBS1: Induced by hepatitis C virus (HCV) to
THBS1 Plays a role in dentinogenesis via its anti- [128,234]
Mechanism unknown. promote the proteolytic activation TGF-β1,
E1 angiogenic properties. Also suggested to
which promotes HCV RNA replication.
play a role in ER stress response.
DYNC1H1:
DYNC1H1: DYNC1H1: Aids in uncoating of HIV-1 nucleocapsids
Subunit of dynein complex. Integral part Postulated to be involved in during infection.
of cellular transport machinery across cells transport of viral structural proteins Proposed to be involved in the transport of
DYNC1H1 [128,235,236]
including neuronal cells. Plays a role in in host cell and implicated in influenza virus X-31, human foamy virus
mitotic spindle and metaphase plate neurological manifestations of (HFV), HIV1 reverse transcription complexes
assembly. CHIKV. (RTC), herpes simplex virus type 1, and
Mokola virus.
ATP1B3 ATP1B3: ATP1B3: ATP1B3: [128,235,236]Viruses 2018, 10, 294 14 of 30
Part of the sodium/potassium-transporting Probably facilitates fusion of viral Shown to inhibit enterovirus 71 (EV71)
ATPase that maintains electrochemical envelope to host membrane during replication by up-regulating type-I interferon
gradient and is important for viral entry. production.
osmoregulation. Proposed to be a pro-viral factor for HIV-1 by
accelerating the degradation of BST-2.
BST-2:
Restricts Lassa virus replication and release.
Restricts viral like particle (VLP) release of the
BST-2:
following viruses: vesicular stomatitis virus
Lipid-raft associated protein that is part of
(VSV), hepatitis C virus (HCV), Kaposi’s
the antiviral response pathway. Blocks the BST-2:
sarcoma-associated herpesvirus (KSHV), [142,143,237–
BST-2 release of many enveloped mammalian Proposed to restrict virus release by
human immunodeficiency virus 1 (HIV-1), 244]
virus by tethering the mature virions to latching onto the CHIKV E1 protein.
ebola virus, Machupo virus MACV) Nipah
the cell plasma membrane of the infected
virus, Zaire ebolavirus (ZEBOV), Lake Victoria
cells.
marburgvirus (MARV), Rift Valley fever virus
(RVFV), cowpox virus (CPXV), and influenza
virus.
BST-2:
BST-2: nsP1 reverses BST-2 ability to restrict BST-2:
nsP1 BST-2 [142,143,156]
See above. virus release by down-regulating the See above.
latter’s expression.
Rpb1:
Rpb1:
Does not get degraded by the Rpb1:
A catalytic subunit of the RNA polymerase
Rpb1 CHIKV nsP2 proteins. Instead is Same observations were found in Sindbis, [169]
II complex that catalyses RNA
degraded via nsP2 mediated Semliki Forest virus (old world alphaviruses).
transcription.
ubiquitination.
SFRS3 (SRp20):
SFRS3/SRp20 (Serine SFRS3 (SRp20):
RNA splicing factor, aids in exon-inclusion SFRS3 (SRp20):
and Arginine Rich Proposed to be crucial for IRES-mediated [175,245]
during alternative splicing. Involved in Mechanism unknown.
Splicing Factor 3) translation in poliovirus.
mRNA nuclear export.
VIM (Vimentin), TACC3 VIM, TACC3, CEP55 and KLC4:
VIM:
nsP2 (transforming, acidic Proposed to be hijacked by nsP2 for
Proposed to be involved in the distribution
coiled-coil containing transport into the infected cells.
VIM, TACC3, CEP55, and KLC4: and acidification of endosomes, allowing
protein 3), CEP55 Interaction with CHIKV nsP3 was [175,246,247]
Cytoskeletal components. successful release of influenza A viral genome.
(centrosomal protein 55 also reported and is proposed to aid
TACC3, CEP55, KLC4:
kDa), and KLC4 (kinesin in the anchorage of the replication
Not reported.
light chain 4) complex.
ASCC2 (activating signal ASCC2:
ASCC2:
cointegrator 1 complex ASCC2, TRIM27, MRF4P1L1, Not reported.
Proposed to regulate/involved in DNA
subunit 2), TRIM27 EWSR1, IKZF1, and ZBTB43: TRIM27: [175,248–250]
transcription and repair.
(tripartite motif 27), Mechanism unknown. Interacts and is degraded by the immediate
TRIM27:
MRFAP1L1/MRG15 early protein ICP0 of the herpes simplex virusYou can also read
